1. Field of the Invention
This invention relates to a ceramic welding process in which oxidising gas and a mixture of refractory and fuel powders are projected against a surface and the fuel is burnt to generate sufficient heat that the refractory powder becomes at least partially melted or softened and a cohesive refractory mass is progressively built up against that surface. The invention also relates to a ceramic welding powder mixture comprising refractory powder and fuel powder, for use in such a ceramic welding process.
2. Description of the Related Art
Ceramic welding processes are useful for the manufacture of new refractory bodies, for example bodies of rather complicated shapes, but in current commercial practice, they are most often used for lining or repairing hot refractory structures such as furnaces or ovens of various kinds, and they enable eroded areas of the refractory structure (provided that those areas are accessible) to be repaired while the structure is substantially at its operating temperature and in some cases even while the structure is still operating. In any event, it is desirable for there to be no deliberate cooling of the refractory structure from its normal operating temperature. The avoidance of such deliberate cooling tends to promote the efficiency of the ceramic welding reactions, avoids further damage to the structure due to thermal stresses set up by such cooling and/or by subsequent reheating to operating temperature, and also helps to reduce furnace "down time".
In ceramic welding repair processes, refractory powder, fuel powder and oxidising gas are projected against the site to be repaired and the fuel is burnt so that the refractory powder becomes at least partially melted or softened and a refractory repair mass is progressively built up at the repair site. The fuel used typically consists of silicon and/or aluminium, though other materials such as magnesium and zirconium may also be used. The refractory powder may be selected so that the chemical composition of the repair mass matches as closely as possible the composition of the refractory to be repaired, though it may be varied, for example so as to deposit a coating of a higher grade refractory on the base structure. In usual practice, the fuel and refractory powders are projected from a lance as a mixture in a stream of oxidising carrier gas.
Due to the intense heat generated on combustion of fuel powders at or close to the surface to be repaired, that surface also becomes softened or melted, and as a result, the repair mass, which is itself largely fused together becomes strongly adherent to the repaired wall, and a highly effective and durable repair results. Previous disclosures of ceramic welding repair techniques are to be found in British Patents No. 1,330,894 and 2,110,200.
Hitherto, one of the most widespread uses of ceramic welding repair processes has been for the refurbishment of coke ovens which are formed from silica refractories. The standard ceramic welding powder most often used for the repair of silica refractories comprises silica together with silicon and optionally aluminium as fuel powder. Silica refractories are in fact the easiest to repair by ceramic welding at least in part because silica refractories are of relatively low refractory grade, so that the temperatures (e.g. 1800.degree. C. or more) reached in the ceramic welding reaction zone easily allow the formation of an adherent cohesive repair mass, and the refractory grade requirements of that repair mass are usually no higher than those of the original silica refractory structure.
We have found, however, that certain problems arise when repairing refractories of higher grades or in other cases when the refractory grade requirements of the ceramic weld mass are particularly stringent. Examples of high grade refractories are: chrome- magnesite, magnesite-alumina, alumina-chrome, magnesite-chrome, chrome, and magnesite refractories, high-alumina refractories, and refractories containing a considerable proportion of zirconium such as CORHART (Trade Mark) ZAC (a fused alumina-zircon-zirconia refractory). For achieving the formation of a ceramic weld mass which has a refractory grade and/or composition approaching or matching those of such high grade refractories, it is not always sufficient to use a standard ceramic welding powder as described above.
A particular problem that arises in the case of a ceramic welding repair mass which is to be subjected to very high temperatures during its working life is the avoidance of a phase within the repair mass which has an insufficiently high softening or melting point. The cohesiveness of a repair mass containing such a phase is impaired at high temperatures and its resistance to corrosion at high temperatures is also not as good as might be hoped for. In general, a refractory phase which is relatively less physically resistant to heat is also more easily attacked chemically at high temperatures.